The invention relates to a hand-operated electric tool, especially a jigsaw, having a motor unit provided in the tool housing, which motor unit contains in a motor housing the stator and the rotor, wherein the rotor shaft projecting with one end from the motor housing is mounted in two bearings fixed in the motor housing, having a gear train which is arranged to be driven by the rotor shaft, which gear train is coupled to the rotor shaft by way of a countershaft that forms an extension of the rotor shaft and is rotatably mounted in a bearing, and having a tool chuck which is arranged to be driven by means of the gear train.
Electric tools, especially jigsaws, in which a pre-assembled motor unit is provided for the drive, are well known. The use of the pre-assembled motor unit, which is manufactured in large piece numbers and therefore economically, has the advantage that the very compact construction of the motor unit enables it to be mounted by fixing the motor housing in the housing of the electric tool, without the rotor and stator having to mounted in separate steps and without any need for the bearings of the rotor shaft to be set in specially constructed locating regions of the housing of the electric tool. With these motor units, however, for many applications, inter alia jigsaws, it is a disadvantage that the portion of the rotor shaft projecting from the motor housing is comparatively short, so that it cannot be coupled directly to the gear train arranged at distance from the motor housing in the housing of the electric tool. On the contrary, a countershaft has to be provided, which acts as an extension of the rotor shaft and has a toothed arrangement, preferably a pinion, which meshes with the input gearwheel of the gear train.
This countershaft has to be rotatably mounted in an additional bearing, and it is normally rigidly connected to the rotor shaft. This produces, however, a three-point bearing of the unit comprising rotor shaft and countershaft, namely, on the one hand in the two bearings of the rotor shaft and on the other hand in the additional bearing of the countershaft. The bearings of such a three-point bearing cannot be coaxially aligned with absolute accuracy, however, so that in operation the rotating unit comprising rotor shaft and countershaft is subject to bending stresses. These present no problems provided that the unit comprising rotor shaft and countershaft is of comparatively stable construction and the stresses acting on this unit in the region in which it engages with the gear train are not too great. As the stresses in the engagement region with the gear train increase, however, for instance in a relatively high-performance jigsaw, in which additional impact stress is transferred, via the gear, from the reciprocating saw blade engaged with the workpiece to the unit comprising rotor shaft and countershaft, it has been found that there is a risk that the three-point mounted unit will fracture.
The invention is based on the problem of eliminating the risk of fracture arising in the known construction.